The first objective of this project is to deveop definitive genetic techniques for use with human lymphocyte cell cultures. Methods for maximizing and controlling lymphocytic intralineal hybridizations will be studied. The resulting fusion hybrids will be analysed for chromosomal stability by a technique which can detect chromosome loss in each cell generation. The second objective of this proposal is to utilize the above methods in an attempt to answer several basic questions: can a reliable determination of the dominance or recessiveness of certain unselected phenotypes to made in lymphocytic fusion hybrids? Does mitotic crossing-over occur naturally in these hybrids? Can it be induced to occur?